Tetrafluoroethylene resins are highly resistant to heat and chemicals, have high electrical insulating properties, provide high water repellency and provide good biocompatibility. Because of these advantages, porous materials of tetrafluoroethylene resins are extensively used in filters, diaphragms, waterproof but airpermeable materials, electrical coatings, sealants, medical equipment and artificial organs. While several methods have been proposed as techniques for producing porous materials of tetrafluoroethylene resin.
The basic principles of a method depending on drawing operations are described in Japanese Patent Publication No. 13560/1967. The first step consists of mixing a powder of tetrafluoroethylene resin with a liquid lubricant and shaping the mixture into a unfired film, tube, or rod by paste extrusion, calender rolling, or a combination of these two methods. The subsequent steps comprises (1) removing the liquid lubricant by evaporation or extraction, (2) drawing the dry shaped product to make it porous, and (3) firing the porous product to a temperature higher than the melting point of the tetrafluoroethylene resin so as to set the porous structure.
A method for producing a porous material of tetrafluoroethylene resin described in U.S. Pat. No. 3,527,616 is as follows. A mixture of a polytetrafluoroethylene dispersion and polymethylmethacrylate is kneaded with preheated rolls, and then injection molded. The product obtained is subjected to a compression molding under heating, and thereafter polymethylmethacrylate is extracted with acetone, as a result, a porous material of tetrafluoroethylene resin is obtained.
Another method for producing a porous material of tetrafluoroethylene resin is described in Japanese Patent Application (OPI) No. 56578/73 (the term "OPI" used herein means a "published unexamined Japanese patent application"). In this method, a polytetrafluoroethylene hydrophilic dspersion is mixed with a fine powdered filler and a pore forming agent, and then molded in a thin film. The product is subjected to a heat treatment, and thereafter the pore forming agent is removed by solvent extraction, as a result, a porous material of tetrafluoroethylene resin is obtained.
Among the above methods, the method depending on drawing operation is preferred because this method is good in pore size uniformity and strength of the product and productivity. However, in the present invention there is no limitation on the methods for producing a porous material of tetrafluoroethylene resin.
The structure of the porous material prepared by this process may vary by some degree, depending upon the draw ratio or other drawing conditions such as temperature and speed, but it basically has a fibrous structure wherein nodes are interconnected by small filaments, providing pores in areas bounded by the filaments and nodes. Generally, by increasing the draw ratio, the filaments are made longer and the nodes smaller, with the result that the proportion of pores or the porosity, is increased.
One serious problem with the thin porous material of tetrafluoroethylene resin prepared by the conventional method is that it has low adhesive properties, which is an inherent defect of tetrafluoroethylene resins.
Tetrafluoroethylene resins have such a high resistance to chemicals and solvents that they are not all reactive with acids, alkalis, or organic solvents. The only substances that are capable of attacking tetrafluoroethylene resins are molten alkali metals, certain solutions of alkali metals, and hot fluorine and chlorine trifluoride. Therefore, one method available today for modifying the surface of materials of tetrafluoroethylene resins so that they can be bonded to other resins is to treat them with an ammonia solution of metallic sodium. For example, U.S. Pat. No. 3,632,387 discloses a method for modifying the surface of materials of tetrafluoroethylene resins by using an ether solution of sodium naphthalate or an ammonia solution of metallic potassium.
Another approach that can be taken consists of drawing a porous material of tetrafluoroethylene resin at a high draw ratio so as to form pores that are large enough to provide anchorage effects to enable physical adhesion.
A porous polytetrafluoroethylene has some adhesive property compared with a non-porous one. A low viscosity resin can penetrate the pores, and generate the anchorage effect. In the case of a porous polytetrafluoroethylene having a high porosity and a large pore diameter due to a high drawing ratio, a resin or a resin solution can penetrate the pore easily. Thus the anchorage effects are increased and the adhesive property is improved.
However, this method, which requires an increase in the overall porosity, and involves great difficulty in producing a porous material having the desired pore size. Further problems arise from the high flexibility of the product, the thin porous material may deform in the bonding step, or the adhesive used will penetrate into the material to an undesirably deep portion. For example, a plurality of tetrafluoroethylene tubes for filter use that have been rendered sufficiently porous to provide adequate adhesive properties cannot be fixed at both ends of each tube with a resin adhesive without collapsing the portions to be bonded during handling, or letting the adhesive penetrate through the walls of tubes to block the interior.
As mentioned above, a thin porous film material made of a tetrafluoroethylene resin can be provided with an increased flexibility and higher degree of bendability by drawing the material at a sufficiently high draw ratio to provide a higher porosity, but at the same time, a tube made of such material tends to easily collapse into a flat form. Methods for avoiding this problem are described in U.S. Pat. Nos. 4,304,010 and 4,306,318, a tubular thin porous film material is drawn at a given ratio and fired, and after inserting a cylindrical metal member through the tube, the latter is heated in a radiant furnace until the outer surface of the tube becomes over-sintered. However, this is not industrially advantageous, since it involves great difficulty in treating the surface of an elongated article in a continuous fashion.